The invention pertains to signal processing and, more particularly, to pulse compression receivers and correlators. The invention has application in RADAR, LIDAR and other range-finding systems of the type employed, by way of non-limiting example, in autonomous vehicles such as self-driving cars, as well as in wireless communications modems of the type employed, by way of non-limiting example, in Massive-MIMO (multiple-in-multiple-out) networks such as 5G wireless telecommunications, all by way of non-limiting example.
Range-finding systems use reflected waves to discern, for example, the presence, distance and/or velocity of objects. Although sound-based ranging has been used in nature for millions of years, mankind did not discover how to harness it and radio frequency-based ranging (RADAR) until the late nineteenth and early twentieth centuries. Laser-based ranging (LIDAR) followed advent of the laser itself, in the 1960's.
Fundamental to automated ranging systems is broadcasting a pulse into the environment and matching it with incoming signals to determine whether they contain reflections of the pulse off objects of potential interest. Though easily stated, the practice of this is anything but. In part, this is because the range-resolution of the reflections is inversely proportional to the transmitted pulse's bandwidth. The higher the bandwidth, the smaller (i.e., the finer) the range-resolution. While this favors short pulses (which tend to be of high bandwidth), they typically result in very complex receiver and transmitter architectures and in limited signal-to-noise ratios. (As those skilled in the art will appreciate, the signal-to-noise ratio (SNR) can be expressed by the relation SNR=Pulse energy/Noise Energy. Noise Energy, in turn, is proportional to the receiver's band-width (k×T×BW). This is why the SNR is weaker for higher band designs.) Longer pulses (which tend to be of lower bandwidth) simplify instrument design and implementation and improve signal-to-noise ratios for the same power levels, yet, with reduced resolution.
Pulse compression is a technique that gets the best of both worlds. By modulating the transmitted signal, e.g., varying the frequency within each pulse or by coding the phase of a continuous-wave signal, this technique can provide the improved signal strength of longer, lower-power pulses with the improved resolution of shorter pulses. For example, by embedding a known a-priori pattern into each pulse, the arrival time of its reflection—and, therefore, the range of the object from which that reflection has occurred—can be resolved with greater precision by finding the point of highest correlation between the pulse pattern and the incoming reflection signals. In other words, very fine range resolution can now be achieved with long pulse durations.
Although it has proven a boon to the art, pulse compression can prove expensive to implement, esp., for example, at speeds necessary to support range finding for commercial autonomous vehicle operation.
In view of the foregoing, an object of the invention is to provide improved methods and apparatus for signal processing.
Related objects are to provide improved methods and apparatus for signal correlation and for pulse compression.
A further related object of the invention is to provide such improved methods and apparatus as can be applied in range-finding, wireless communications and other applications.
A further object of the invention is to provide improved such methods and apparatus as are suitable for use with RADAR, LIDAR and other range-finding technologies.
A still further object of the invention is to provide improved methods and apparatus for transmitting and receiving pulses and their reflections in such range-finding systems.
Still yet another object of the invention is to provide an improved correlator and methods of operation thereof for use with such range-finding and other systems.
Yet still another object of the invention is to provide such an improved correlator and methods as are reconfigurable.